Faulty Bone Formation

A disease that can be debilitating in young horses is osteochondrosis. This affliction has its genesis during the youngster's growing years and can compromise the horse's ability to perform later in life if it is not dealt with early. Osteochondrosis is one of the diseases that comes under the general umbrella of developmental orthopedic disease (DOD), a term coined in 1986 by a blue ribbon panel of researchers sponsored by the American Quarter Horse Association (AQHA). DOD encompasses all general growth disturbances in the young horse and includes the following: Osteochondrosis, acquired angular limb deformities, physitis, subchondral bone cysts, flexural deformities, cuboidal bone abnormalities, and juvenile osteoarthritis.

Osteochondrosis is a frustrating affliction because, while there are a great many theories bandied about as to its cause, there is no strong scientific evidence that any one factor is responsible. Rather, it appears, the cause could be multi-factoral and can range from genetics to mineral imbalance.

"It is definitely a disease of growth," says Gayle Trotter, DVM, MS, Dipl. ACVS, an associate professor of surgery at Colorado State University (CSU). "It is most apt to show up in young horses six to 12 months of age and sometimes when the horse is a long yearling."

Genetics, he feels, is implicated. He notes that there have been instances where a number of foals sired by a particular stallion developed osteochondrosis.

Diet can also be a factor, Trotter believes. "A diet that is high in phosphorus and high in energy has been implicated in some experiments."

At this point, we should pause and discuss just what osteochondrosis and a subset condition called osteochondritis dissecans (OCD) are, and how the conditions are manifested.

How Bones Grow

Rather than provide an in-depth explanation of what occurs during bone growth, we will get back to basics with layman's terminology using information borrowed from Trotter and C. Wayne McIlWraith, BVSc, FRCVS, PhD, Dipl. ACVS, Dipl. ECVS, Director of the Orthopaedic Research Center at CSU.

When bones grow, an ossification or hardening process is involved. This means that cartilage is turned into bone. Osteochondrosis occurs when this basic process is interrupted or assaulted in any way. The result can be lesions or pockets within the bone itself. These are spots where cartilage fails to convert into bone during the growing process, and thus they become weak spots. These spots can be responsible for a manifestation of the disease known as OCD. This specific malady can truly compromise a horse's ability to perform.

"OCD is a disease subset of osteochondrosis," says Trotter. "This condition affects the joint cartilage and often also involves the subchondral bone just beneath the cartilage surface."

The weakened spot of cartilage, or cartilage and bone, sometimes separates from the main bone and becomes a free body or "joint mouse," Trotter says. "In most cases, however, the fragments remain loosely attached to their bone of origin, but the debris that is released into the joint from beneath the flap results in synovitis or joint inflammation, and results in the clinical signs of pain and lameness that are seen with the disease."

Joints Commonly Affected

OCD can affect many joints, he says, but the two most commonly involved are the hock and stifle. Next in line of occurrence are the fetlock and shoulder joints, but they are much less frequently affected.

The disease is not breed-specific. It seems to show up frequently in Standardbreds, Trotter says, but is also commonly found in Thoroughbreds, Arabians, Quarter Horses, and even draft horses. Although multiple joints can be involved, this is unusual. Normally, Trotter says, it will be in just one joint type.

In one study of 161 horses with stifle OCD done at CSU, reports Trotter, only five had OCD affecting the rear fetlocks, four had hock OCD, and one had OCD of the shoulder.

However, that being said, Trotter nevertheless points out that it is not all that unusual for opposing or bilateral joints to be involved--such as both hock joints or both stifle joints. So, if the disease shows up in one joint, the same joint in the opposing leg should be examined carefully.

Trotter says that when the disease strikes the hock, it appears to be more prevalent among Thoroughbreds. About 60% of the time, it will occur when the horse is a year of age or less.

The afflicted horse, he explains, generally develops swelling in the joint and lameness. The lameness can vary from mild to severe, and he adds that some of the more severely affected horses will have a "bunny hop" action behind that might be confused at the onset with a neurological problem.

The most consistent sign of OCD in the stifle, Trotter explains, is joint distention (swelling). Radiographs provide the most useful information to the diagnostician, he adds. Normally, the treatment of choice for stifle OCD is surgery, and quite often it is highly successful (more on this later).

"In one study of 252 stifle joints in 161 horses, follow-up information was available for 134 horses," said Trotter. "Of the 134 horses, 64% returned to their previous use, 7% were in training, 16% were unsuccessful, and 13% were unsuccessful due to reasons unrelated to the stifle." Trotter says that the success rate was higher in horses with smaller lesions.

Another likely spot for OCD is in the hock joint. When this is the case, the Standardbred breed seems to be more at risk, although hock OCD has been found in a number of other breeds as well. The afflicted horse often will have swelling from joint fluid accumulation along the medial or inside aspect of the joint. Sometimes the horse is lame, Trotter says, but often he is not. If the horse involved is a racehorse, the condition often is seen at two years of age, but with non-racehorses, it more frequently is found in yearlings before they enter training.

Arthroscopic surgery often is the treatment of choice. Abnormal cartilage is surgically cleaned from the joint surface and any loose cartilage or bone fragments removed.

"Surgery should be considered early enough in the course of the disease," says Trotter, "so that the joint capsule is not unduly stretched, making resolution of the joint effusion (swelling from the leaking fluid) less likely."

The prognosis for recovery is good, he feels. "In a study involving 183 horses, 76% raced successfully or performed at their intended use after surgery."

When OCD strikes a fetlock joint--front or rear--it most commonly involves fragmentation and irregularity at the lower or distal end of the cannon bone. It can also involve the first phalanx or long pastern bone. Treatment protocols can range from a conservative approach involving rest to surgery. The prognosis for recovery is generally favorable, but is dependent on the severity of the condition.

OCD of the shoulder joint, says Trotter, is probably the most debilitating type affecting horses. "Generally, large areas of the joint surfaces are involved and secondary joint disease is common. However, it is unusual to have free or loose bodies develop. OCD of the shoulder is less common than for the other joints described and seems to affect Quarter Horses and Thoroughbreds with a similar incidence."

Conservative treatment involving rest generally is recommended, Trotter says, with surgery sometimes being called for. However, he adds, "The shoulder is probably the most difficult joint on which to perform arthroscopic surgery, due to the depth of the joint below the muscles in the area." Prognosis for full recovery of shoulder OCD often is guarded at best.

You Are What You Eat

As Trotter has mentioned, there likely are multiple factors involved in the onset of OCD in young horses, but diet does seem to be at or near the forefront.

McIlwraith, in a treatise published in the wake of a seminar on lameness at CSU in 1995, had this to say about the role of nutrition and its relationship to osteochondrosis: "There has been an increased incidence of OCD lesions noted in horses fed 130% of what the National Research Council recommends for carbohydrates and protein. This study by Dr. Kate Savage (BVSc, MS, PhD, Dipl. ACVIM), which was very well controlled, showed that high-energy diets (120% more than National Research Council requirements) consistently produced lesions of osteochondrosis in weanling foals compared to a control diet based on 100% NRC requirements. Some people have focused on 'high protein' being the problem, but this has not been demonstrated."

A number of research projects since 1995 have borne out the contention that a diet overly high in energy definitely is implicated in the onset of OCD. There are other factors that can also be involved, such as trauma, too much zinc or too little copper in the diet, and, as was mentioned at the outset, genetics.

A take-home message for horse owners would seem to be this: Feed a growing horse a balanced diet and don't push the high-energy feeds. Make sure the horse ingests the proper balance of minerals. If OCD shows up in several foals by the same stallion, consider that genetics might be the culprit. And, most importantly, if a young horse shows signs of lameness or swelling in any joint, call your veterinarian.

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